One known method of producing a blank of an optical fibre consists firstly in vapour phase deposition of a layer of one type of glass including a doping material for increasing its refractive index on the inside cylindrical surface of a glass tube made of silica for example, then reducing the tube so as to remove its axial cavity and reduce its outside diameter. This reduction can be obtained by making the tube turn about its axis on a glass maker's lathe and by locally heating the rotating tube so as to soften it and by moving the heated zone along the tube progressively from one end of the tube to the other.
A rod is thus obtained whose diameter is less than the outside diameter of the original tube. This rod is constituted by a silica cladding and an axial core. The average refractive index of the core, which is formed by the deposited layer of glass, is higher than that of the cladding, which is itself formed by the silica of the tube. This rod is a blank which can be drawn in a furnace to obtain an optical fibre, as is well known.
Optical fibres obtained from such blanks have characteristics which can be insufficient for allowing the transmission of light signals at a high rate. For example, high dispersion of the transmitted signals may be observed: the duration of a very short signal injected at the input of such a fibre is much increased at the output.
The present invention aims to improve the method of producing a blank of an optical fibre set forth hereinabove, so as to improve the transmission characteristics of the optical fibres produced from this blank.